It is known that the refining of pig iron in a converter involves burning carbon which is contained in the pig iron in a quantity up to 4% as well as other oxidizable accompanying elements in the blown-in air or in the blown-in oxygen. The heat produced in this process can be utilized for melting sponge iron. The capacity of the converter for sponge iron to be melted is limited, however, in an undesirable manner by the carbon content and the content of oxidizable accompanying elements in the pig iron in the converter and in the substances to be melted. Moreover, the nozzles with which the oxygen is introduced into the molten pig iron are subject to heavy mechanical, thermal and chemical stresses which lead to malfunctions in the refining and melting process. Many attempts have therefore been made to introduce larger quantities of heat into the converter by suitable measures and to reduce the process-inherent stresses to which the oxygen feed devices are subjected. In one such prior art method, the metal bath in the converter has been heated by an oil heating system operated with oxygen, but this method has not found acceptance because the capacity of the converter for sponge iron to be melted could not be substantially increased in view of economical considerations due to the insufficient heat transfer from the combustion gases to the metal bath. In another prior art method, pieces of calcium carbide have been added to the melt, but this method does not yield a significant increase in the melting capacity, again in view of economical considerations.
It is further known that gaseous and liquid hydrocarbons have been used in refining processes to protect the oxygen nozzles and the bottom of the converter. These hydrocarbons are introduced into the melt through nozzles associated with the oxygen nozzles, then decompose in an endothermic reaction at the temperature of the melting bath and thus effect cooling of the oxygen nozzles and of the converter bottom. The oxygen and the hydrocarbons can be blown in through jacket nozzles. Although the melting capacity of the converter may be increased by the blowing in of hydrocarbons, significant drawbacks result. The gaseous pyrolysis products, especially hydrogen, flow so fast through the melt that a part thereof is still not burnt when they reach the converter exhaust and thus little heat is developed from the added amount of hydrocarbon. Further, part of the hydrogen dissolves in the metal melt and must be removed therefrom by a costly cleaning process by rinsing with suitable gases, for example nitrogen. In addition, the pyrolysis products are extremely explosive if they contain a certain proportion of oxygen and thus endanger the safety of the entire system. For this reason, only small quantities of hydrocarbons can be blown into the converter which, of course, results in a limitation of the melting capacity of the converter.
In the past, it has been generally required to feed the pieces or pellets of sponge iron, which may be replaced in part by scrap, into the converter in charges and to remove the melt discontinuously so that longer start and dead times result for the converter. During discontinuous operation utilization of the exhaust gases is not very economical because large portions of the exhaust gases developing at the beginning and at the end of the melting process must be discarded.
The particular difficulties encountered during melting of sponge iron are generally caused by its poor heat conductivity. For example, in an electric arc furnace, the major proportion of the metallic material may be sponge iron (up to 80%), which produces higher melting costs due to the poor heat conductivity of the sponge iron. The high intensity of movement of the melt in the discontinuous melting process does have a favorable influence during refining with oxygen on the melting capacity for sponge iron, but the meltable quantity of sponge iron is limited to about 30% by the energy users contained in the pig iron. Moreover, because of the poor heat conductivity of the sponge iron, clods of sponge iron which are difficult to melt may develop and lead to difficulties Particularly during the removal of slag.